Without a doubt the soundboard is the most important piece of wood effecting the sound of the instrument. Even with great skills it will be very difficult if not impossible to make a great guitar with mediocre wood and other materials. With that said selecting a top with good sound producing qualities is paramount to making a great sounding guitar.
As a guitar maker always striving to build the best sounding instrument possible, one must begin with the best wood possible. If you are inexperienced at guitar making go around and look at as many soundboards as you can. Most suppliers will let you check out the wood in person. Look at the different grades and see for yourself what the differences are as far as visual, touch and the sound it makes as you tap it with your knuckle. Does the wood make a musical sound or is it a dull thud? Does the sound sustain and trail off or does it just die out quickly? If you are short on experience it may be a good idea to purchase only master grade material since it will be difficult at this point to find a real bargain in the lesser grades of wood.
Developing Your Guitar Wood Intuition
Developing a relationship between the wood you are using and the results you are looking for is paramount to successful guitar making. Your intuition in guitar making is an important asset and it needs to be developed, or more accurately encouraged. Most people already have some ability for accurate observation using their senses - all that is needed is the confidence to listen and be guided by it. As your experience working with the material grows, you are unconsciously creating and storing a collection of information. This information will become the basis for the decisions you make as a guitar builder.
Wood for the Soundboard
Quarter sawn wood is more dimensionally stable than any other cut of wood. It also has a very calm and pleasing appearance in its straight vertical grain, which runs parallel to the length of the piece vertical to the front and back faces. A true quarter sawn cut of wood will reveal rays which are intricate little cross stripes running perpendicular to the vertical grain lines on the face known as silk (Pictured here on a cedar top). Seeing these rays is an indication that the top is perfectly quarter sawn. If properly cut, there is a much better chance that quarter sawn wood will not warp or twist. As the moisture content in the air and wood changes, quarter sawn wood will change dimension to a lesser degree than other cuts of wood. Therefore almost all the wood used to build a guitar is quarter sawn especially the soundboard and neck.
Selecting good pieces for your top can be challenging at first especially if you do not have much experience with wood. Your first criteria is visual. Look for quarter sawn wood with silk as described above. Also look for flaws such as bark inclusions, knots and so on. Check that these fall outside the boundaries of the top. If the knot is small and without voids I may allow it on the top. The wood around a knot is always stiffer than it is anywhere else on the board. Again its a personal choice.
Width of the grain lines is not as important as people think. Stiffness and weight are more important. The wood used for a top should be stiff. The stiffer the better. Feeling the stiffness in a top can be deceiving. To compare tops they should be the same thickness. So if you remember a top that was super stiff you must also remember how thick is was because the thicker the top the stiffer it will be. If the two tops are different thicknesses you are comparing apples and oranges.
The same goes for pitch. When you tap the top it will resonate. The sound it makes will have a pitch. The thicker the top the higher the pitch so again all tops must be at the same thickness and size to compare. Much has been said about tap tone and whether or not it is a good prediction method for a good sounding top. Everyone seems to have an opinion. For me, the sustain is most important. If the tap tone has little sustain, I will not use it. I always look for good sustain and the higher pitches. Again I am comparing my tops at the same thickness and overall size so the comparison is valid. I will always choose a top that has both sustain and a higher resonate pitch. Also, contrary to popular belief stay away from tops with heavy amounts of bear claw. It has been my experience that tops with tons of bear claw just don't sound that good. They are visually striking of course but its all about sound right? I find that if the top has just a hint of bear claw it will work better on instruments.
If you are new to all of this my best advise is to get your hands on as many tops as you can. Tap them, feel them, notice the rays and after using the wood make mental notes on the sound. Picking out the right wood is a process that will take time to develop. You simply cannot ask someone to tell you what is good and what is not, you need to find out for yourself and your guitars. Good luck and enjoy the process.